“High-temperature and high-power THz quantum cascade lasers”

The highest temperature performance in terahertz QCLs has been demonstrated by using active regions based on the resonant-phonon depopulation scheme along with metalmetal waveguides to provide low-loss confinement, which have been used to demonstrate maximum temperatures of 164 K in pulsed mode and 117 K in cw mode. It is expected that there are two primary mechanisms which reduce the population inversion at high temperatures: the thermal activation of electron-LO-phonon scattering from the upper to lower state, and the thermal backfilling of the lower state by carriers from the injector. The use of a double-phonon depopulation mechanism has been successfully used in midIR QCL designs to suppress the latter effect and obtain room temperature, cw performance. It is natural to attempt such a scheme in the terahertz. We present experimental results from several schemes for implementing double-resonant-phonon depopulation. Initial results do not reveal any improvement in maximum operation temperature compared to single-phonon designs – in at one case the modified design introduced an additional parasitic current channel which depleted the upper radiative state. Our results suggest that thermal backfilling is not yet a limiting factor in terahertz QCL performance.